Determining the position or location of a wireless emitter (or transmitter) is a well-known problem with both military and commercial applications. Many techniques have been proposed and applied to this problem over the last 70 years. Most of these techniques are based on the assumption that the wireless signal travels from the source to the receiving antennas along the line-of-sight (LOS) path connecting them. The classical position location techniques, direction-of-arrival (DOA), time-of-arrival (TOA) and differential-time-of-arrival (DTOA), are all based on this assumption and exploit it. The localization is done via triangulation, using several such measurements.
In recent years there has been a growing interest in location determination of emitters in urban canyons and in indoor venues where LOS conditions usually do not exist. In these cases the propagation from the wireless emitter to the receiving antennas usually undergoes reflections from buildings and walls, referred to as multipath. Consequently, the multipath signals arriving at the receiving antennas may be very different from the LOS path. As a result, the classical position location techniques are not valid.
Fingerprinting techniques have been developed to overcome this multipath problem. Fingerprinting algorithms are based on the premise that there is a one-to-one correspondence between the emitter location and the signal characteristics of the received multipath signals, i.e., that a fingerprint (or signature) can be extracted from the signal and serve as a unique identifier of the location. The localization problem is casted as a pattern recognition problem, namely, a database of fingerprints is pre-collected in the desired area to be covered, and the location is determined by comparing the extracted fingerprint to the fingerprint database.
Two types of fingerprinting techniques have been developed about the same time. The first is described by Wax et al. in U.S. Pat. Nos. 6,026,304, 6,064,339, 6,112,095, and 6,249,680, which are incorporated herein by reference. This technique is based on using the multipath characteristics coherently received by a multiple-antenna base station (BS) as the location fingerprint. The other fingerprinting technique is described by Bahl and Padmanabhan in “RADAR: an in-building RF-based user location and tracking system”, INFOCOM 2000. Nineteenth Annual Joint Conference of the IEEE Computer and Communications Societies. Proceedings. IEEE, 2000, pp. 775-784 vol. 2. This technique is based on using the received signal strength (RSS) obtained at several BSs as the location fingerprint.
The RSS fingerprint suffers from dependence on many irrelevant parameters such as the orientation of the transmitter and body shadowing, but more critically, it suffers from high signal strength variability caused by constructive and destructive interference between the multipath signals. As a result, the accuracy of this technique is limited. To provide reasonable accuracy, this approach requires the combination of signal strength measurements from multiple BSs. In many cases, however, multiple BSs may not be able to receive the signals, in which case the accuracy is very low.
The coherently received multipath-based fingerprint, on the other hand, exploits the multipath to its advantage, rather than suffering from it, thus enabling much better accuracy. Moreover, it can provide good accuracy with only a single BS. Prior work on this approach, however, was mostly confined to the narrowband signals of the advanced mobile phone system (AMPS) and used only the directions-of-arrival information for creating the fingerprint. Although it was extended to wideband signals used in code division multiple access (CDMA) systems, it was done by exploiting the power delay profile, a feature unique to CDMA signals, as an additional, separate fingerprint.